1. Field of the Invention
The invention relates to the removal of residual product from a vessel and more specifically to the removal of residual product from a filter-dryer unit with a side opening for emptying, employing a so called “lasso-technique”.
2. Background of the Invention Art
Filter-Dryers are used in the production for filtration and drying at the first place. But also extractions, washing and other intermediate processes are processed in these apparatus. It often is the aim to process several steps in the same unit. If it is possible to proceed without discharging the product and thus without opening the containment, losses of valuable substances and danger for poisoning personnel or damaging the product can be minimized.
The variations of Materials to be treated that way go from simple chemical products which have to be dried to highly sensitive and even toxic products of the pharmaceutical Industry. Especially for the latter it is desirable not to expose the products to air and endanger personnel by handling it. To treat several production steps in one and the same unit therefore is highly desirable.
The basic material is typically fed to the filter-dryer unit as a slurry of a predefined mixture of crystals suspended in liquid. A batch of such a slurry is commonly loaded into the filter-dryer. This filter-dryer unit usually is in the form of a sealed, cylindrically shaped vessel having a filter in its base to allow the liquid to be drained from the vessel through the filter plate. A side or center discharge. opening is provided to allow the resulting dry product, often a powder, to be removed from the vessel to a receiving container or the like. Usually a mechanical agitator rotates in the vessel at a relatively slow rate. This mechanical agitator will be moved up and down during its rotation in order to generate new surfaces on the slurry and powder especially during the drying process. With this agitator one avoids the generation of wet clumps as a most homogeneous mass is required in the slurry-phase as well as in the powder-phase. This mechanical agitator also serves to remove the dried or treated product mechanically out of the vessel discharge opening.
In this apparatus the liquid particles will be separated from the solids by a first step with a filtration process. The bottom plate of such a apparatus is a filter plate and a following drying process under special temperature and vacuum conditions. For this processes the target always is to produce crystals and solids as product with the highest possible yield, e.g. the lowest possible moisture contained.
Treating cosmetics, pharmaceutical and other valuable goods the problem faced is always that a residue of the finished product tends to remain in various parts of the vessel. U.S. Pat. No. 5,564,350 introduces a so called “gasknife” for the removal of these residues by means of pressed air or pressed inert gases. For many products this device works very good. It might give some problems if the fine fluffy powder does not create e.g. explosion danger or the wet powder does stick too hard to the filter plate on the bottom of the apparatus.
The main part and in many cases the last remaining volume of residual material, called the heel, is left on the bottom plate of the vessel and is not readily removable for discharge. This bottom plate of the vessel is provided with relatively fragile and expensive filter screens which allow the liquid to pass from the vessel while preventing the loss of solids by holding it back. The mechanical agitator is a relatively coarse device which may be on the order of up to 4 meters in diameter. To ensure that the filter-screens are not damaged by the agitator, a minimal clearance is required between the agitator and the screens. This clearance may be on the order of up to several millimeters for big filter-dryers. The residue in that space and elsewhere in the vessel is typically washed away at the end of the batch processing and is therefore a loss which reduces the yield of the production.
In the processing of pharmaceutical substances different batches may not be mixed together and therefore the residue is essentially lost if it has to be washed out. The lost residue often makes up to 15% of the batch. For expensive material such as highly concentrated pharmaceuticals substances this is great loss. Very often the heel will be scratched out by hand which might be dangerous for the personnel due to poisonous or toxic volatiles or substances.
Due to this fact the use of a filter-dryer for several process steps might not be applicable even if the process itself offers very advantageous possibilities for the product with such procedures.